Method of handover and route diversity in mobile radio communication

ABSTRACT

A method of handover and route diversity in a mobile radio communication which is less time consuming and capable of improving the frequency spectrum utilization efficiency and securing the high quality of service regardless of the moving speed of the mobile station. In the method, a loop transmission line by which each base station is connected with neighboring base stations is provided; a handover information is transmitted through the loop transmission line, where the handover information is relayed by each base station from one of the neighboring base stations to another one of the neighboring base station; and the handover of a communication of a mobile station from one traffic channel of one base station to an idle traffic channel of another base station is carried out by using the handover information transmitted through the loop transmission line. The method can also be applied to a route diversity reception.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of handover and routediversity operations in a mobile radio communication system such as anautomobile telephone system.

2. Description of the Background Art

In a mobile radio communication system, the efficient utilization oflimited frequency spectrum is one of the major technical problems. Forthis reason, a so called cellular system is adopted in general. In thiscellular system, the efficient frequency spectrum utilization isachieved by providing a plurality of mutually distanced base stationswithin a service area of the system so as to enable a so calledco-channel reuse which is a repetitive use of the same frequency atdifferent base stations, where the base stations are sufficientlydistanced from each other to avoid a co-channel interference. In such acellular system, an area covered by each base station is called a cell.

In this cellular system, a higher frequency spectrum utilizationefficiency can be achieved by making a radius of each cell smaller,because it becomes possible for the same frequency to be repetitivelyused at shorter distances. Such a use of smaller size cells is effectivein increasing a system capacity regarding a number of subscribers thatcan be handled by the system.

However, as a consequence of an increased number of cells required forcovering the entire service area, such a use of smaller size cells alsonecessitates an increased number of so called handover operationsrequired to be performed when a mobile station moves from one cell toanother during a single continuous communication. As a result, thecapacities and sizes of each base station and a central control stationcontrolling the base stations will be increased considerably andconfigurations of the base stations and the central control station willinevitably become quite complicated.

Namely, in the cellular system, as a mobile station which iscommunicating through one particular base station of one particular cellmoves from that particular cell to a neighboring cell, the receivinglevel at that particular base station will gradually decreases, so thatin order to continue this communication it is necessary to switch thebase station through which the mobile station communicates from thatparticular base station of that particular cell to the base station ofthe neighboring cell. Here, in switching the base station, an activeland transmission line connecting the central control station with thebase station needs to be switched, while the frequency used for thecommunication by that mobile station needs to be switched from thetraffic channel used in an original cell from which the mobile stationis moving out to that of an idle traffic channel available in the basestation of a destination cell to which the mobile station is moving in.Such an operation is known as the handover operation (which is sometimesalso referred to as a handoff operation).

More specifically, as shown in FIG. 1, a conventional cellular systemcomprises a central control station 1, and a plurality (four in FIG. 1)of base stations 2 to 5 controlled by the central control station 1through land transmission lines 11 to 14, respectively, where a mobilestation 10 moves through a plurality (four in FIG. 1) of cells 6 to 9associated with the base stations 2 to 5, respectively. In this cellularsystem, the central control station 1 is connected to an ordinarytelephone network (not shown), such that a communication to and from themobile station 10 can be transmitted through one of the base stations 2to 5, the central control station 1, and the ordinary telephone network.

Now, assuming that the mobile station 10 is located in a cell 8 andcommunicating through the base station 4, as the mobile station 10 movesto the neighboring cell 7, the handover operation will proceed asfollows.

(1) As a distance between the base station 4 and the mobile station 10becomes greater than the cell radius, the receiving level at the basestation 4 decreases below the specific level. When this decrease of thereceiving level is detected by the base station 4, the base station 4requests the handover of the communication of the mobile station 10 tothe central control station I through the land transmission line 13.

(2) In response to this request for the handover from the base station4, the central control station 1 commands the neighboring base stations2, 3, and 5 to monitor the transmitted radio waves from the mobilestation 10 through the land transmission lines 11, 12, and 14,respectively.

(3) In response to this command from the central control station 1, eachof the base stations 2, 3, and 5 receives the transmitted radio waves ofthe traffic channel used for the communication of the mobile station 10specified by the command from the central control station 1, measuresthe receiving level for this traffic channel, and reports the measuredreceiving level to the central control station 1.

(4) Then, the central control station 10 selects the base station forwhich the reported receiving level is the highest among the basestations 2, 3, and 5. In this exemplary case, the base station 3 willhave the highest receiving level as the mobile station 10 is moving intothe cell 7.

Next, the central control station 10 commands the selected base station3 to report its idle traffic channel #N which is available for thehandover of the communication of the mobile station 10.

(6) In response to this command from the central control station 1, thebase station 3 reports the idle traffic channel #N to the centralcontrol station 1.

(7) Then, the central control station I commands the base station 4 tosend a command signal for the handover to the traffic channel #N to themobile station 10. Meanwhile, the central control station 1 alsocommands the base station 3 to activate the transmitter-receiver for thetraffic channel #N, while switching the active land transmission linefrom that connected to the base station 4 to that connected to the basestation 3.

(8) In response to the command from the central control station 1, thebase station 4 sends the command signal for the handover to the mobilestation 10.

(9) In response to the command signal from the base station 4, themobile station 10 switches its traffic channel to the traffic channel #Nspecified by the command signal, so as to establish the communicationthrough the base station 3.

Now, in addition to the increase of capacities and sizes of each basestation and the central control station and the complication ofconfigurations of the base stations and the central control stationalready mentioned above, such a conventional method of handover in thecellular system has a drawback that it is quite time consuming as itrequires several signal exchanges between the central control station 1and the base stations after the lowering of the receiving level isdetected at the base station 4 in order to complete the handoveroperation.

As a consequence, in a case the cell radius is made smaller, it becomespossible for the mobile station to move further onto the nextneighboring cell before the handover operation from one cell to itsneighboring cell is completed, such that the highly undesirable failureof the handover of the communication could occur in the cellular system.To eliminate such a possibility for the failure of the handover, thecell radius cannot be made smaller than a certain limit value, and thislimitation on the cell radius in turn creates an upper limit to theefficient frequency spectrum utilization.

Moreover, when the mobile stations with higher moving capability suchautomobile telephones and the mobile stations with lower movingcapability such as portable telephones are involved in such aconventional cellular system together, the cell radius must bedetermined in accordance with the automobile telephones which can movefaster, in order to prevent the occurrence of the failure of thehandover, even though the cell radius so determined is not capable ofachieving the highest frequency spectrum utilization efficiency for theportable telephones. Consequently, the base station such as that locatedin a heavily populated area where a number of portable telephonesubscribers is large needs to have a large number of traffic channelsassigned, but this in turn lowers the frequency spectrum utilizationefficiency further.

On the other hand, when the cell radius is determined in accordance withthe portable telephones, it becomes difficult for the system to handlethe frequent handover operations required by the communication of theportable telephone used on a fast moving object such as an automobile.

In addition, as the cell radius becomes smaller, an area betweenneighboring cells which is not well served by any cell may appear, andalso the quality of service will be lowered drastically as soon as themobile station moves out of the area covered by the cells, such thatthere is a great possibility for the subscribed portable telephone userto suffer from the poor quality of service.

Now, in a mobile radio communication, the radio transmissioncharacteristic is quite adverse because of the large receiving levelfluctuation and other causes, such that the occurrence of errors in thetransmission signals is unavoidable. For this reason, the mobile radiocommunication system utilizes various error correction mechanisms.However, these error correction mechanisms are effective only withrespect to the instantaneous fluctuation of the receiving level, and notvery effective with respect to the relatively gradual variation of thereceiving level caused by the shadowing of the transmission path due tothe terrain and environmental features.

To such a relatively gradual variation of the receiving level, a socalled route diversity reception in which the transmitted radio wavesreceived by a plurality of base stations are utilized is known to beeffective. For instance, the automobile telephone of NTT (NipponTelegraph and Telephone) Corporation utilizes the base station routediversity reception for the control channel.

More specifically, a conventional route diversity reception is achievedas follows.

Namely, a conventional mobile radio communication system utilizing theroute diversity reception has a configuration shown in FIG. 2, where thesystem includes a central control station 21 and a plurality of basestations 22 to 28 each of which is connected with the central controlstation 21 through land transmission lines 29 to 35, respectively.

In this mobile radio communication system, a signal transmitted fromeach of the base stations 22 to 28 to the central control station 21 hasa format shown in FIG. 3 which includes a transmission signal 36 from amobile station and an encoded receiving level information 37 indicatingthe receiving level at each base station. In other words, each basestation relays the transmission signal 36 from the mobile station to thecentral control station 21 by attaching the receiving level information37 to the transmission signal 36.

The central control station 21 then selects the base station for whichthe receiving level is the highest among all the base stations 22 to 28,and uses the transmission signal 36 transmitted from this selected basestation as the transmission signal 36 received from the mobile station.

Such a conventional mobile radio communication system has drawbacks thatthe expensive land transmission line must be provided between thecentral control station 21 and each one of a plurality of the basestations and that the control function of the central control station 21inevitably becomes complicated as a consequence of utilizing the routediversity reception.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodof handover in a mobile radio communication which is less time consumingsuch that the smaller cell radius can be used so as to improve thefrequency spectrum utilization efficiency, and capable of securing thehigh quality of service regardless of the moving speed of the mobilestation.

It is also an object of the present invention to provide a method ofroute diversity in a mobile radio communication which can be realizedless expensively, without requiring additional control function whichcomplicates the central control station.

According to one aspect of the present invention there is provided amethod of handover in a mobile radio communication using a cellularsystem formed by a plurality of base stations, the method comprising thesteps of: providing a loop transmission line by which each base stationis connected with neighboring base stations; transmitting a handoverinformation through the loop transmission line, where the handoverinformation is relayed by each base station from one of the neighboringbase stations to another one of the neighboring base station; andcarrying out the handover of a communication of a mobile station fromone traffic channel of one base station to an idle traffic channel ofanother base station by using the handover information transmittedthrough the loop transmission line.

According to another aspect of the present invention there is provided amethod of handover in a mobile radio communication using a cellularsystem formed by a plurality of control base stations each of which iscovering a cell within a service area of the system, a plurality of basestations each of which is covering a mini-cell within a cell area of thecell covered by each of the control base stations, the method comprisingthe steps of: providing a loop transmission line by which a control basestation and each base station are connected with neighboring basestations; transmitting a handover information through the looptransmission line, where the handover information is relayed by eachbase station from one of the neighboring base stations to another one ofthe neighboring base station; and carrying out the handover of acommunication of a mobile station from one traffic channel of one basestation to an idle traffic channel of another base station by using thehandover information transmitted through the loop transmission line whena mobile station moves from one mini-cell to another mini-cell.

According to another aspect of the present invention there is provided amethod of handover in a mobile radio communication using a cellularsystem formed by a plurality of base stations, the method comprising thesteps of: providing a loop transmission line by which each base stationis connected with neighboring base stations; monitoring at a mobilestation a receiving quality at each of the base stations in a controlchannel during idle periods of a currently used traffic channel in orderto determine a destination base station for the handover; transmitting ahandover information through the loop transmission line, where thehandover information is relayed by each base station from one of theneighboring base stations to another one of the neighboring basestation; and carrying out the handover of a communication of a mobilestation from the currently used traffic channel of a currently used basestation to an idle traffic channel of the destination base stationdetermined by the mobile station by using the handover informationtransmitted through the loop transmission line.

According to another aspect of the present invention there is provided amethod of route diversity in a mobile radio communication using acellular system formed by a central control station and a plurality ofbase stations, the method comprising the steps of: providing aninterstation transmission line by which each base station is connectedwith neighboring base stations and to which the central control stationis connected at an end in a signal transmission direction; receivingradio waves from a mobile station and measuring a receiving quality forthe radio waves from the mobile station at each base station; andtransmitting a receiving quality information indicating a currentlyhighest receiving quality obtained by the base stations and atransmission signal representing the radio waves from the mobile stationreceived at the highest receiving level, the receiving level informationand the transmission signal being updated at each base station such thatthe receiving quality information received by the central controlstation is the highest receiving quality among the receiving qualitiesobtained by all the base stations.

Other features and advantages of the present invention will becomeapparent from the following description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a conventional mobile radiocommunication system in a form of a cellular system.

FIG. 2 is a schematic diagram of a conventional mobile radiocommunication system utilizing a route diversity.

FIG. 3 is a diagram of a format for a signal transmitted from each basestation to a central control station in the system of FIG. 2.

FIG. 4 is a schematic diagram of a first embodiment of a mobile radiocommunication system in a form of a cellular system using a method ofhandover according to the present invention.

FIG. 5 is a diagram of a format for an original base station signaltransmitted from each base station through a loop transmission line inthe system of FIG. 4.

FIG. 6 is a schematic diagram of a configuration of each base station inthe system of FIG. 4.

FIG. 7 is a diagram of one format for a destination base station signaltransmitted from each base station through the loop transmission line inthe system of FIG. 4.

FIG. 8 is a diagram of another format for a destination base stationsignal transmitted from each base station through the loop transmissionline in the system of FIG. 4.

FIG. 9 is a diagram of another format for an original base stationsignal and a destination base station signal transmitted from each basestation to the loop transmission line in the system of FIG. 4.

FIG. 10 is a schematic diagram of a second embodiment of a mobile radiocommunication system in a form of a cellular system using a method ofhandover according to the present invention.

FIG. 11 is a diagram of a format for an original base station signaltransmitted from each base station through a loop transmission line inthe system of FIG. 10.

FIG. 12 is a schematic diagram of a configuration of each base stationin the system of FIG. 10.

FIG. 13 is a diagram of a format for a receiving level report signaltransmitted from each base station through the loop transmission line inthe system of FIG. 10.

FIG. 14 is a diagram of a format for an idle traffic channel signaltransmitted from each base station through the loop transmission line ina third embodiment of a mobile radio communication system in a form of acellular system using a method of handover according to the presentinvention.

FIG. 15 is a timing diagram for a case of relaying the idle trafficchannel signal of FIG. 14 at each base station.

FIG. 16 is a schematic diagram of a fourth embodiment of a mobile radiocommunication system in a form of a cellular system using a method ofhandover according to the present invention.

FIG. 17 is a schematic diagram of a part of the system of FIG. 16 in oneexemplary situation for explaining the handover operation in the system.

FIG. 18 is a schematic diagram of a part of the system of FIG. 16 inanother exemplary situation for explaining the handover operation in thesystem.

FIG. 19 is a timing diagram for an operation of a mobile station in afifth embodiment of a mobile radio communication system in a form of acellular system using a method of handover according to the presentinvention.

FIG. 20 is a diagram of a format for a control station signaltransmitted through the loop transmission line in the system of thefifth embodiment.

FIG. 21 is a diagram of a format for a mobile station signal transmittedthrough the loop transmission line in the system of the fifthembodiment.

FIG. 22 is a diagram of a format for a control channel signaltransmitted through the loop transmission line in the system of thefifth embodiment.

FIG. 23 is a flow chart for a first procedure of the handover operationin the system of the fifth embodiment.

FIG. 24 is a flow chart for a second procedure of the handover operationIn the system of the fifth embodiment.

FIG. 25 is a flow chart for a third procedure of the handover operationin the system of the fifth embodiment.

FIG. 26 is a flow chart for a fourth procedure of the handover operationin the system of the fifth embodiment.

FIG. 27 is a flow chart for a fifth procedure of the handover operationin the system of the fifth embodiment.

FIG. 28 is a schematic diagram of a sixth embodiment of a mobile radiocommunication system in a form of a cellular system using a method ofroute diversity according to the present invention.

FIG. 29 is a schematic diagram of a configuration of each base stationin the system of FIG. 28.

FIG. 30 is a timing diagram for an operation of a base station toachieve the route diversity reception in the system of FIG. 28.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 4, a first embodiment of a mobile radiocommunication system using a method of handover according to the presentinvention will be described in detail.

In this first embodiment, the system comprises a central control station101 and a plurality (four in FIG. 4) of base stations 102 to 105, whichare connected through a loop transmission line formed by a transmissionline element 115 connecting between the central control station 101 andthe base station 102, a transmission line element 116 connecting betweenthe base stations 102 and 103, a transmission line element 117connecting between the base stations 103 and 105, a transmission lineelement 118 connecting between the base stations 105 and 104, and atransmission line element 119 connecting between the base station 104and the central control station 101.

In this system of the first embodiment, while the mobile station 110 iscommunicating through one of the base station, that base station outputsan original base station signal to this loop transmission line of thesystem in a format shown in FIG. 5, where the original base stationsignal includes a signal identification 120 identifying the type of thesignal transmitted (i.e., that it is the original base station signal),a base station identification 121 identifying the base station fromwhich this original base station signal is transmitted (i.e., that basestation itself), a traffic channel number 122 indicating the trafficchannel currently used for the communication of the mobile station 110at that base station, and a receiving level information 123 indicatingthe receiving level of the transmitted radio waves from the mobilestation 110 at that base station.

Each of the base stations 102 to 105 has a schematic configuration shownin FIG. 6, where the base station comprises an antenna 124 fortransmitting and receiving the radio waves to and from the mobilestation 110, a transmitter-receiver 125 for supplying the transmissionsignal to the antenna 124 and receiving the transmitted signal from theantenna 124, a control circuit 126, a transmission line element 128 forreceiving signals from an upper station through the loop transmissionline, and a transmission line element 129 for transmitting signals to alower station through the loop transmission line. Here, each of the basestations 102 to 105 has a predetermined number of traffic channelsassigned according to the traffic demand of each cell area.

In this configuration, the control circuit 126 of each base stationgenerates the signal identification 120, the base station identification121, the traffic channel number 122, and the receiving level information123 for one of the traffic channel through which the mobile station 110is currently communicating, and outputs the original base station signalin a format of FIG. 5 to the loop transmission line of the systemthrough the transmission line element 129.

On the other hand, from the transmission line element 128, the similaroriginal base station signal in a format of FIG. 5 outputted by theupper station is transmitted to each base station. Each base stationrelays such original base station signals transmitted from the upperstation to the loop transmission line through the-transmission lineelement 129.

In addition, in this system, for each base station, the base stationswhich are neighboring this base station such that the handover from thisbase station may occur are determined in advance, and the controlcircuit 126 of each base station memorizes the base stations for whichthis base station has such neighboring relationships.

When the original base station signal in a format of FIG. 5 transmittedthrough the loop transmission line is received through the transmissionline element 128, each base station determines whether this originalbase station signal is transmitted from one of the base stations withwhich it is in the neighboring relationships. If so, the base stationstarts receiving the traffic channel specified by the traffic channelnumber 122 of that original base station signal and measures thereceiving level for that traffic channel.

In a case the measured receiving level is higher than the receivinglevel indicated by the receiving level information 123 of that originalbase station signal, the base station judges that the mobile station 110has moved into the cell area of its own cell. In this case, the basestation outputs a destination base station signal indicating its ownbase station identification, its own idle traffic channel number, andthe base station identification of the original base station from whichthe mobile station 110 is moving out. Here, in a case there is no idletraffic channel available in that base station, the destination basestation signal will not be generated.

This destination base station signal can be in a format shown in FIG. 7.In this format shown in FIG. 7, the destination base station signalincludes a signal identification 130 identifying the type of the signaltransmitted (i.e., that it is the destination base station signal), abase station identification 131 identifying the base station from whichthis destination base station signal is transmitted (i.e., that basestation itself), an idle traffic channel number 132 indicating the idletraffic channel available at that base station, an original base stationidentification 133 identifying the original base station from which themobile station 110 is moving out, and an original traffic channel number134 indicating the traffic channel used for the communication of themobile station 110 at the original base station.

Alternatively, the destination base station signal may be in a formatshown in FIG. 8, in which the destination base station signal in thefirst format of FIG. 7 is attached behind the original base stationsignal in a format of FIG. 5 such that the original base station signaland the destination base station signal are transmitted together whenthe original base station signal is to be relayed to the looptransmission line. Here, the signal identification 130 may be omitted asit is meaningless in this format, and the original base stationidentification 133 and the original traffic channel number 134 may beomitted as they overlap with the base station identification 121 and thetraffic channel number 122.

Alternatively, the destination base station signal may be in a formatshown in FIG. 9, where the destination base station signal includes thesignal identification 120 identifying the type of the signaltransmitted, the base station identification 121 identifying the basestation from which this destination base station signal is transmitted,the traffic channel number 122 indicating the traffic channel currentlyused for the communication of the mobile station 110, a measuring basestation identification 135 identifying the base station at which thehighest receiving level for the traffic channel specified by the trafficchannel number 122 is measured up to then, a measured receiving levelinformation 136 indicating the receiving level of the transmitted radiowaves from the mobile station 110 at the base station identified by themeasuring base station identification 135, and the idle traffic channelnumber 132 indicating the idle traffic channel available at the basestation identified by the measuring base station identification 135.

When this format of FIG. 9 is adopted, the original base station outputsthe original base station signal also in this format of FIG. 9, wherethe measuring base station identification 135 is identical to the basestation identification 121 initially, and the receiving levelinformation 123 in the format of FIG. 5 is replaced by the measuredreceiving level information 136 in the format of FIG. 9.

Then, at each base station which receives the original base stationsignal in a format of FIG. 9, the receiving level for the trafficchannel specified by the traffic channel number 122 is measured andcompared with the receiving level indicated by the measured receivinglevel information 136. When the receiving level measured at that basestation is less than the receiving level indicated by the measuredreceiving level information 136, the transmitted original base stationsignal is relayed to the loop transmission line without any change asthe destination base station signal at that base station. On the otherhand when the receiving level measured at that base station is greaterthan the receiving level indicated by the measured receiving levelinformation 136, the measuring base station identification is changed tothe base station identification of that base station, and the measuredreceiving level information 136 is changed to the receiving levelmeasured at that base station, while the idle traffic channel number 132is registered, before being transmitted as the destination base stationsignal to the loop transmission line.

When such a destination base station signal returns back to the originalbase station after circulating around the loop transmission line, theoriginal base station determines the destination base station and theidle traffic channel in this destination base station from thedestination base station signal, and transmits a handover command signalto the mobile station 110 so as to command the mobile station 110 tocarry out the handover of its communication to the idle traffic channelof the destination base station determined from the destination basestation signal.

Here, it is noted that in this first embodiment, each base station doesnot assign the traffic channel which is registered as the idle trafficchannel number 132 in the destination base station signal to anycommunication until the final determination of the destination basestation and the handover traffic channel.

Now, assuming that the mobile station 110 is communicating through thebase station 104, and moving out from the cell area of this base station104, the base station 104 generates and outputs the original basestation signal described above to the loop transmission line. Then,assuming that the base station 103 received the radio waves from themobile station 110 at the highest receiving level and reported its idletraffic channel #N in the destination base station signal which issubsequently transmitted to the original base station 104, the handoveroperation will proceed as follows.

(1) The base station 104 notifies the central control station 101through the loop transmission line that the base station 103 isdetermined as the destination base station.

(2) In response to this notification from the base station 104, thecentral control station 101 notifies the base station 103 through theloop transmission line that it is determined as the destination basestation for the handover, while also notifying the other base stationsthrough the loop transmission line that they are not determined as thedestination base station for the handover.

(3) The base station 104 transmits a handover command signal to themobile station 110 so as to command the mobile station 110 to carry outthe handover of its communication to the idle traffic channel #N of thedestination base station 103.

(4) Then, the base station 103 activates the transmitter-receiver forthe traffic channel #N, while notifying the central control station 101through the loop transmission line that the handover of thecommunication of the mobile station 110 from the base station 104 to thetraffic channel #N of the base station 103 has been completed.

In this first embodiment, when the idle traffic channel registered by acertain base station is rewritten at another subsequent base station,that certain base station cannot know that the idle traffic channel itregistered has been rewritten at a time of rewriting. However, each basestation can recognize whether the idle traffic channel it registered isgoing to be the handover traffic channel or not as the central controlstation 101 notifies each base station as to whether it is determined asthe destination base station for the handover or not.

Thus, according this first embodiment, it becomes possible to establisha so called decentralized autonomous control in the cellular system inwhich each base station monitors the traffic channels of the other basestations, judges whether the mobile station is moving into its own cellarea, and transmits its idle traffic channel to the original basestation without any command from the central control station, such thatthe handover operation can be carried out less time consumingly comparedwith a conventional cellular system without increasing the capacitiesand sizes of each base station and the central control station andcomplicating the configurations of the base stations and the centralcontrol station. As a consequence, it becomes possible to use thesmaller cell radius so as to improve the frequency spectrum utilizationefficiency, and to secure the high quality of service regardless of themoving speed of the mobile station.

Referring now to FIG. 10, a second embodiment of a mobile radiocommunication system using a method of handover according to the presentinvention will be described in detail.

In this second embodiment, the system comprises a central controlstation 201 and a plurality (four in FIG. 10) of base stations 202 to205, which are connected through a loop transmission line formed by atransmission line element 215 connecting between the central controlstation 201 and the base station 202, a transmission line element 216connecting between the base stations 202 and 203, a transmission lineelement 217 connecting between the base stations 203 and 205, atransmission line element 218 connecting between the base stations 205and 204, and a transmission line element 219 connecting between the basestation 204 and the central control station 201.

In addition, the base stations 202 to 205 are directly connected withthe central control station 201 through land transmission lines 211 to214, respectively, besides the loop transmission line.

In this system of the second embodiment, while the mobile station 210 iscommunicating through one of the base station, that base station outputsan original base station signal to this loop transmission line of thesystem in a format shown in FIG. 11, where the original base stationsignal includes a receiving level detection request 220 indicating therequest for monitoring the receiving level, a base stationidentification 221 identifying the base station from which this originalbase station signal is transmitted (i.e., that base station itself), anda traffic channel number 222 indicating the traffic channel currentlyused for the communication of the mobile station 210 at that basestation.

Each of the base stations 202 to 205 has a schematic configuration shownin FIG. 12, where the base station comprises an antenna 224 fortransmitting and receiving the radio waves to and from the mobilestation 210, a transmitter-receiver 225 for supplying the transmissionsignal to the antenna 224 and receiving the transmitted signal from theantenna 224, a control circuit 226, a transmission line element 227 fortransmitting and receiving signals to and from the central controlstation 201 through the land transmission line, a transmission lineelement 228 for receiving signals from an upper station through the looptransmission line, and a transmission line element 229 for transmittingsignals to a lower station through the loop transmission line. Here,each of the base stations 202 to 205 has a predetermined number oftraffic channels assigned according to the traffic demand of each cellarea.

In this configuration, the control circuit 226 of each base stationmonitors the receiving level of the traffic channel used for thecommunication of the mobile station 210 and recognize that the mobilestation 210 is moving out from its cell area so that the handoveroperation is necessary when the receiving level decreases below thespecific level, in which case the handover operation is requested to thecontrol circuit 226. In response, the control circuit 226 generates thereceiving level detection request 220, the base station identification221, and the traffic channel number 222 for one of the traffic channelthrough which the mobile station 210 is currently communicating, andoutputs the original base station signal in a format of FIG. 11 to theloop transmission line of the system through the transmission lineelement 229.

On the other hand, from the transmission line element 228, the similaroriginal base station signal in a format of FIG. 11 outputted by theupper station is transmitted to each base station. Each base stationrelays such original base station signals transmitted from the upperstation to the loop transmission line through the transmission lineelement 229.

Thus, the original base station signal transmitted from the originalbase station to the loop transmission line will subsequently be relayedthrough the base stations 202 to 205 and the central control station 201until it returns back to the original base station.

In addition, in this system, for each base station, the base stationswhich are neighboring this base station such that the handover from thisbase station may occur are determined in advance, and the controlcircuit 226 of each base station memorizes the base stations for whichthis base station has such neighboring relationships. For instance, inFIG. 10, the base station 203 has the neighboring relationships with thebase station 202, 204, and 205, while the base station 205 has theneighboring relationships with the base station 203 and 204.

When the original base station signal in a format of FIG. 11 transmittedthrough the loop transmission line is received through the transmissionline element 228, each base station determines whether this originalbase station signal is transmitted from one of the base stations withwhich it is in the neighboring relationships. If so, the base stationstarts receiving the traffic channel specified by the traffic channelnumber 222 of that original base station signal and measures thereceiving level for that traffic channel. For instance, in FIG. 10, ifthe original base station is the base station 202, the base stations 203and 204 will be judged as the neighboring base stations of the originalbase station 202 so that they will start measuring the receiving levelfor that traffic channel as soon as the original base station signal isrelayed to them, whereas the base station 205 will be judged as not aneighboring base station of the original base station 202, so that itwill only relay the original base station signal to the looptransmission line.

The base station which is judged as the neighboring base station of theoriginal base station then outputs a receiving level report signal in aformat shown in FIG. 13, where the receiving level report signalincludes a signal identification 229 identifying that it is thereceiving level report signal, a monitoring base station identification230 identifying the base station which measured the receiving level(i.e., that base station itself), a monitored traffic channel number 231indicating the traffic channel monitored by that base station, and areceiving level code 232 indicating the receiving level obtained for themonitored traffic channel in an encoded form.

The central control station 201 compares the receiving level indicatedby the receiving level code 232 of the receiving level report signalstransmitted through the loop transmission line from the base stations,and selects the base station for which the receiving level indicated bythe receiving level code 232 In the receiving level report signal is thehighest as the destination base station for the handover.

Hereafter, the central control station 201 controls the cellular systemsubstantially similarly to the case of the handover operation in aconventional cellular system.

Thus, according this second embodiment, it becomes possible to establisha partially decentralized autonomous control in the cellular system inwhich each base station monitors the traffic channels of the other basestations without any command from the central control station, such thatthe handover operation can be carried out less time consumingly comparedwith a conventional cellular system without increasing the capacitiesand sizes of each base station and the central control station andcomplicating the configurations of the base stations and the centralcontrol station. As a consequence, it becomes possible to use thesmaller cell radius so as to improve the frequency spectrum utilizationefficiency, and to secure the high quality of service regardless of themoving speed of the mobile station.

Referring now to FIGS. 14 and 15, a third embodiment of a mobile radiocommunication system using a method of handover according to the presentinvention will be described in detail.

In this third embodiment, the system configuration is substantially thesame as that of the second embodiment shown in FIG. 10. Also, each ofthe base stations has a configuration substantially the same as that ofthe second embodiment shown in FIG. 12.

Now in this third embodiment, each base station outputs an idle trafficchannel signal to the loop transmission line of the system in a formatshown in FIG. 14, where the idle traffic channel signal includes a basestation identification 320 identifying the base station from which thisidle traffic channel signal is transmitted (i.e., that base stationitself), an idle traffic channel number 322 indicating the idle trafficchannel available at that base station, and an idle traffic channelreservation 321 indicating a presence or an absence of a reservation forthe idle traffic channel indicated by the idle traffic channel number322.

Thus, the control circuit 226 of each base station regularly generatesthe base station identification 320, the idle traffic channelreservation 321 which is initially set to indicate the absence of thereservation, and the idle traffic channel number 322 indicating the idletraffic channel available at that base station at a time of thegeneration of the idle traffic channel signal, and outputs the generatedidle traffic channel signal in a format of FIG. 14 to the looptransmission line of the system through the transmission line element229. In a case there is no idle traffic channel available at that basestation at a time of generation of the idle traffic channel signal, thisabsence of the available idle traffic channel is indicated by the idletraffic channel number 322.

On the other hand, from the transmission line element 228, the similaridle traffic channel signal in a format of FIG. 14 outputted by theupper station is transmitted to each base station. Each base stationrelays such idle traffic channel signals transmitted from the upperstation to the loop transmission line through the transmission lineelement 229.

In relaying the idle traffic channel signal from the other base station,each base station may change the idle traffic channel reservation 321 toindicate the presence of the reservation by inserting its own basestation identification whenever it is judged that the idle trafficchannel indicated by the idle traffic channel number 322 is needed forthe handover of the communication of the mobile station 210 which iscurrently communicating through that base station.

Namely, as shown in FIG. 15, when the base station receives the idletraffic channel signal with the base station identification 320₁, idletraffic channel reservation 321₁, and idle traffic channel number 322₁from the upper station through the transmission line element 228, thecontrol circuit 226 of the base station determines whether it isnecessary to read the base station identification 320₁ and the idletraffic channel reservation 321₁ of this idle traffic channel signal,i.e., whether the idle traffic channel indicated by this idle trafficchannel signal is needed for the handover at that base station. In acase it is determined to be unnecessary to read them, the controlcircuit 226 transmits this idle traffic channel signal unchanged, byturning the base station identification 320₁, idle traffic channelreservation 321₁, and idle traffic channel number 322₁ into the basestation identification 320₂, idle traffic channel reservation 321₂, andidle traffic channel number 322₂ for the idle traffic channel signal tobe transmitted to the lower station through the transmission lineelement 229. On the other hand, when it is determined to be necessary toread them, the control circuit 226 transmits this idle traffic channelsignal by turning the base station identification 320₁ into the basestation identification 320₂, and idle traffic channel number 322₁ intoand idle traffic channel number 322₂ for the idle traffic channel signalto be transmitted to the lower station through the transmission lineelement 229, while replacing the idle traffic channel reservation 321₁by the idle traffic channel reservation 321₂ in which the presence ofthe reservation is indicated by the inserted base station identificationof that base station.

Thus, the idle traffic channel signal transmitted from each base stationto the loop transmission line will subsequently be relayed through thebase stations 202 to 205 and the central control station 201 whileregistering the reservation for the idle traffic channel whenever theneed arises until it returns back to the base station which generatedthis idle traffic channel signal.

Here, each base station does not assign the traffic channel which isregistered as the idle traffic channel number 322 in the idle trafficchannel signal to any communication until the idle traffic channelsignal returns back after circulating through the loop transmissionline.

Now, assuming that the mobile station 210 is communicating through thebase station 204, and moving out from the cell area of this base station204 such that a need for the handover from the base station 204 arises,the handover operation will proceed as follows.

(1) As a distance between the base station 204 and the mobile station210 becomes greater than the cell radius, the receiving level at thebase station 204 decreases below the specific level. When this decreaseof the receiving level is detected by the base station 204, the basestation 204 requests the handover of the communication of the mobilestation 210 to the central control station 201 through the landtransmission line 213.

(2) In response to this request for the handover from the base station204, the central control station 201 commands the neighboring basestations 202, 203, and 205 to monitor the transmitted radio waves fromthe mobile station 210 through the land transmission lines 211, 212, and214, respectively.

(3) In response to this command from the central control station 201,each of the base stations 202, 203, and 205 receives the transmittedradio waves of the channel used for the communication of the mobilestation 210 which is specified from the central control station 201, andmeasures the receiving level which is then reported to the centralcontrol station 201.

(4) Then, the central control station 201 selects the base station forwhich the reported receiving level is the highest among the basestations 202, 203, and 205. In this exemplary case, the base station 203will have the highest receiving level.

Here, it is noted that these four steps (1) to (4) are substantially thesame as the conventional handover operation procedure. In this thirdembodiment, however, these steps (1) to (4) are followed by thefollowing steps (5) to (8).

(5) The central control station 201 commands the base station 204 tocarry out the handover of the communication of the mobile station 210 tothe base station 203. Meanwhile, the central control station 201 alsoswitches the active land transmission line from that connected to thebase station 204 to that connected to the base station 203, and notifiesthe base station 203 through the loop transmission line that it isdetermined as the destination base station for the handover.

(6) In response to the command for the handover from the central controlstation 201, the base station 204 selects the idle traffic channelsignal having the base station identification 320 indicating the basestation 203 among the idle traffic channel signals circulating in theloop transmission line, and reads the idle traffic channel number 322 ofthe selected idle traffic channel signal so as to recognize the idletraffic channel #N for the handover. Then, the base station 204 changesthe idle traffic channel reservation 321 of this idle traffic channelsignal to indicate the presence of the reservation for the idle trafficchannel #N by inserting its own base station identification to the idletraffic channel reservation 321, and then relays this idle trafficchannel signal to the lower station to the loop transmission linethrough the transmission line element 229.

(7) The base station 204 transmits a handover command signal to themobile station 210 so as to command the mobile station 210 to carry outthe handover of its communication to the idle traffic channel #N of thebase station 203.

(8) When the idle traffic channel signal whose idle traffic channelreservation 321 has been changed by the base station 204 returns back tothe base station 203 which generated this idle traffic channel signal,the base station 203 recognizes from the idle traffic channelreservation 321 of this idle traffic channel signal that the basestation 204 is going to use the idle traffic channel #N for thehandover, and activates the transmitter-receiver for the traffic channel#N, while notifying the central control station 201 through the looptransmission line that the handover of the communication of the mobilestation 210 is going to be carried out from the base station 204 to thetraffic channel #N of the base station 203, so as to complete thehandover operation.

The base station 203 subsequently generates and outputs the idle trafficchannel signal again, in which the traffic channel other than #N will beregistered as the idle traffic channel.

In a case the idle traffic channel signal returns back to the basestation which generated this idle traffic channel signal without anyreservation for the idle traffic channel registered and this idletraffic channel is still available at that base channel, that basestation may outputs the same idle traffic channel signal again to theloop transmission line, or the new idle traffic channel signal in whichthe idle traffic channel is replaced by another idle traffic channelavailable.

In a case one traffic channel of one destination base station happens tobe potentially usable by two original base stations, the original basestation which receives the idle traffic channel signal from thedestination base station first will make the reservation for the idletraffic channel, so that by the time the other original base stationreceives this idle traffic channel signal from the destination basestation, the idle traffic channel is no longer available and thereforethis other original base station have to wait for the other idle trafficchannel signal in which the idle traffic channel is not yet reserved.Thus, in this third embodiment, the conflict between two base stationsover a single idle traffic channel of the other base station can beavoided.

Actually, the idle traffic channel reservation 321 is not absolutelyindispensable in this third embodiment. In a case the idle trafficchannel reservation 321 is not used in the idle traffic channel signal,it becomes possible for two original base stations requiring thehandover at the same time to assign the same traffic channel of onedestination base station to two different mobile stations. However, evenIn such a case the conflict between two base stations over a single idletraffic channel of the other base station can also be avoided by meansof a loop check operation as follows.

Namely, in general, the mobile station for which a new traffic channelis assigned as a result of the handover carries out an operation calleda loop check in order to confirm the correctness of the assignment ofthe new traffic channel. This loop check operation is carried out byexchanging a loop cheek signal including a mobile station number betweenthe destination base station and the mobile station, and thecommunication through a traffic channel assigned by the handoveroperation cannot be started until this loop check operation issuccessfully completed. Thus, even when more than one original basestations assign the same traffic channel of one destination base stationto two different mobile stations at the same time, only the mobilestation which completed the loop check with the destination base stationfirst can actually start communicating through the new traffic channel,and the other mobile station which inevitably fails to complete the loopcheck operation will have to wait for the handover to another trafficchannel of the destination base station, so that the conflict betweentwo base stations over a single idle traffic channel of the other basestation can also be avoided.

Thus, according this third embodiment, it becomes possible to establisha partially decentralized autonomous control in the cellular system inwhich each base station monitors the idle traffic channels of the otherbase stations without any command from the central control station, suchthat the handover operation can be carried out less time consuminglycompared with a conventional cellular system without increasing thecapacities and sizes of each base station and the central controlstation and complicating the configurations of the base stations and thecentral control station. As a consequence, it becomes possible to usethe smaller cell radius so as to improve the frequency spectrumutilization efficiency, and to secure the high quality of serviceregardless of the moving speed of the mobile station.

Referring now to FIG. 16, a fourth embodiment of a mobile radiocommunication system using a method of handover according to the presentinvention will be described in detail.

In this fourth embodiment, the entire service area of the system iscovered by a plurality of cells such as a cell 416. Each cell 416 has acontrol base station 415 which covers the entire cell area of the cell416, and a plurality of base stations 417 to 420 covering mini-cells 421to 424 formed within the cell area of the cell 416. The control basestation 415 and the base stations 417 to 420 are connected through aloop transmission line 425 such that the mini-cells 421 to 424 aremanaged substantially as in the first embodiment described above withthe control base station 415 playing a role of a central controlstation. Regions within the cell 416 which are not covered by themini-cells 421 to 424 are covered by the control base station 415 alone.Here, each of the base stations 417 to 420 has a predetermined number oftraffic channels assigned according to the traffic demand within eachmini- cell, while the control base station 415 also has a predeterminednumber of traffic channels assigned according to the traffic demandwithin the cell 416.

In addition, the control base stations such as the control base station415 are connected with a system control station 414 through landtransmission lines such as a land transmission line 426, such that thehandover between the control base stations are managed by the systemcontrol station 414.

In this system, each of the base stations 417 to 420 has a limitedfunctional capacity compared with the control base station 415, so thatwhen an excessively low receiving level or an excessively largereceiving level fluctuation beyond the functional capacity of each basestation is detected for a communication of a mobile station at the basestation, the handover of the communication is carried out from the basestation to the idle traffic channel of the control base station 415.

Namely, as shown in FIG. 17, when the mobile station 429 located withinthe mini-cell 421 and making a communication 431 through the basestation 417 is moving out of the mini-cell 421 by moving along an arrow433, the receiving levels reported from the other base stations 418 to420 through the loop transmission line 425 indicate that the receivinglevels are not high enough, so that the control base station 415commands the handover to its own idle traffic channel to the basestation 417 and the mobile station 429 through the loop transmissionline 425.

Similarly, when the mobile station moving within the mini-cells 421 to424 is moving very fast, the receiving levels reported from the otherbase stations 418 to 420 through the loop transmission line 425 indicatethat the receiving levels are excessively fluctuating, so that thecontrol base station 415 commands the handover to its own idle trafficchannel to the base station and the mobile station through the looptransmission line 425.

Also, when the mobile station 430 located outside the mini-cells 421 to424 and making a communication 432 through the control base station 415is moving out of the cell 416 to the neighboring cell by moving along anarrow 434, the handover is carried out between the control base station415 and the control base station of the neighboring cell under thecontrol of the system control station 414.

On the other hand, while the mobile station is communicating through thecontrol base station 415, the base stations 417 to 420 monitor thiscommunication and report the receiving level to control base station415, such that when the receiving level at one of the base stations 417to 420 becomes sufficiently high for the capacity of that base station,the handover of the communication is carried out from the control basestation 415 to that base station.

Namely, as shown in FIG. 18, when the mobile station 435 located outsidethe mini-cells 421 to 424 and making a communication 438 through thecontrol base station 415 is moving into the mini-cell 417 by movingalong an arrow 441, the handover is carried out from the control basestation 415 to the idle traffic channel of the base station 417.

When the mobile station 436 located within the mini-cell 421 and makinga communication 439 through the base station 417 is moving into theneighboring mini-cell 422 by moving along an arrow 442, the control basestation 415 commands the handover from the base station 417 to the basestation 418 as in the first embodiment described above.

Here, however, when the mobile station moving within the mini-cells 421to 424 is moving very fast, the receiving levels reported from the otherbase stations 418 to 420 through the loop transmission line 425 indicatethat the receiving levels are excessively fluctuating, so that thecontrol base station 415 continues to maintain the communication of themobile station without commanding the handover to the base stations 417to 420.

Thus, according to this fourth embodiment, the mobile stationcommunicates through the base station of a mini-cell as much aspossible, so that the improved frequency spectrum utilization efficiencycan be achieved. In addition, the mini-cells are enclosed within alarger size cell covered by a control base station, such that theregions around the mini-cells and the fast moving mobile station can behandled by the control base station having a larger capacity. Therefore,it becomes possible in this fourth embodiment to provide a method ofhandover in a mobile radio communication capable of securing the highquality of service regardless of the moving speed of the mobile station.

Referring now to FIG. 19, a fifth embodiment of a mobile radiocommunication system using a method of handover according to the presentinvention will be described in detail.

In this fifth embodiment, the system configuration is substantially thesame as that of the first embodiment shown in FIG. 4. Also, each of thebase stations has a configuration substantially the same as that of thefirst embodiment shown in FIG. 6.

This fifth embodiment concerns with a case of utilizing a 3-channel TDMA(time division multiple access) method, in which a decentralizedautonomous control is realized in the cellular system by a mobilestation monitoring the receiving levels of the base stations without anycommand from the central control station in order to carry out thehandover operation.

Here, each of the base stations has a predetermined number of trafficchannels assigned according to the traffic demand of each cell area, anda control channel which is either common to all the base stations ordifferent for each base station. The co-channel reuse of these trafficchannels and the control channel is possible in this system.

In the control channel, the set up of calls to and from the mobilestation to assign the traffic channel to be used is controlled, and eachbase station notifies the control channel numbers of the neighboringbase stations to the mobile station. The mobile station selectivelymonitors the control channel of the highest receiving level at any givenmoment.

In this system, the mobile station determines a cell in which it islocated by monitoring the receiving levels of the base stations, andnotifies the determined located cell information to the base stations inorder to carry out the handover operation, by operating in a mannershown in FIG. 19.

Namely, in the traffic channel, the mobile station operates in asequence of the mobile station transmission time slots 500, 502, and 504in the traffic channel in which the mobile station transmits the radiowaves to the currently used base station through which it iscommunicating, and the mobile station reception time slots 501, 503, and505 in the traffic channel in which the mobile station receives thesignals from that currently used base station.

In addition, the mobile station switches to the control channel at idleperiods of the traffic channel such as a period between the mobilestation reception time slot 501 and the mobile station transmission timeslot 502, and operates in the neighboring base station receiving levelmonitoring time slots 506 and 507 in the control channel in which thereceiving levels of the control channels of the neighboring basestations are monitored. In a case the control channel of each basestation is different from that of the other base stations, the controlchannels of the neighboring base stations specified in the controlchannel of the currently used base station are sequentially monitored ineach neighboring base station receiving level monitoring time slot. Onthe other hand, in a case the common control channel is used for thetime division multiple access by all the base stations, the time slot issequentially switched after the mobile station switches to the controlchannel.

Then, the mobile station selects the neighboring base station for whichthe monitored receiving level is the highest among all the neighboringbase stations, and compares the receiving level of the currently usedbase station with the receiving level of the selected neighboring basestation. When the receiving level of the neighboring base station ishigher than that of the currently used base station, the mobile stationjudges that it has moved to a cell of that selected neighboring basestation. When this moving into a cell of the neighboring base station isdetected, the mobile station switches to the control channel of thatselected neighboring base station and transmits the destination basestation notification signal at the located cell information notificationtime slot 508.

In this system, the central control station outputs the control stationsignal in a format shown in FIG. 20 to the loop transmission line, wherethe control station signal includes an addressed base station number 509indicating a base station from which this control station signal is tobe transmitted to the mobile station, a control signal 510, and atransmission signal 511 to be transmitted to the mobile station in thetraffic channel. This control station signal is sequentially relayedthrough the loop transmission line by the base stations to the addressedbase station indicated by the addressed base station number 509 fromwhich this control station signal is transmitted to the mobile station.

On the other hand, when the base station receives the signal from themobile station, the base station outputs a mobile station signal in aformat shown in FIG. 21, where the mobile station signal includes arelay base station number 512 indicating a base station at which thismobile station signal is received from the mobile station, a controlsignal 513, and a transmission signal 514 to be transmitted to thecentral control station. This mobile station signal is sequentiallyrelayed through the loop transmission line by the base stations to thecentral control station.

Also, the control channel signal in a format shown in FIG. 22 istransmitted through the loop transmission line, where the controlchannel signal includes a generating station number 515 indicating astation from which this control channel signal is generated, anaddressed station number 516 indicating a station to which this controlchannel signal is to be transmitted, and a control signal 517. Here, thegenerating station and the addressed station can includes any of thebase stations as well as the central control station and the mobilestation.

In this fifth embodiment, the handover operation can be carried out byany one of the following five procedures according to the need.

The first procedure shown in FIG. 23 is for a case in which the controlchannels of the base stations are different each other.

In this first procedure, first at the step 519, the mobile stationmeasures the current receiving level of the currently used base stationthrough which it is communicating at the mobile station reception timeslot, while measuring the receiving levels of the control channels ofthe neighboring base stations by switching to the control channel duringthe idle periods in the traffic channel and determining the selectedneighboring base station which has the highest neighboring base stationreceiving level.

Then, at the step 520, the mobile station judges whether it has movesinto a cell of the neighboring base station or not by comparing thehighest neighboring base station receiving level and the currentreceiving level. When the highest neighboring base station receivinglevel is higher than the current receiving level, it is judged that themobile station has moved to a cell of the selected neighboring basestation at the step 520, and next at the step 521 the mobile stationswitches to the control channel of the selected neighboring base stationduring the idle period in the traffic channel and transmits thedestination base station notification signal at the located cellinformation notification time slot, whereas otherwise the step 519 isrepeated.

When the destination base station receives this destination base stationnotification signal in the control channel at the step 522, next at thestep 523, the destination base station selects an idle traffic channelavailable, and outputs the control channel signal of FIG. 22 to the looptransmission line, with itself as the generating station, the originalbase station as the addressed base station, and the control signalindicating the handover to the selected idle traffic channel.

When the original base station receives this control channel signalthrough the loop transmission line at the step 524, the original basestation transmits the handover command signal indicating the idletraffic channel of the destination base station specified in the controlchannel signal to the mobile station in the currently used trafficchannel at the step 525, and then releases the currently used trafficchannel at the step 526 so as to stop relaying the communication of themobile station.

When the mobile station receives this handover command signal from theoriginal base station at the step 527, the mobile station switches itstraffic channel to the idle traffic channel of the destination stationat the step 528, while the destination base station starts relaying thecommunication of the mobile station by assigning the idle trafficchannel at the step 529.

The second procedure shown in FIG. 24 is also for a case in which thecontrol channels of the base stations are different each other.

In this second procedure, first at the step 530, the mobile stationmeasures the current receiving level of the currently used base stationthrough which it is communicating at the mobile station reception timeslot, while measuring the receiving levels of the control channels ofthe neighboring base stations by switching to the control channel duringthe idle periods in the traffic channel and determining the selectedneighboring base station which has the highest neighboring base stationreceiving level.

Then, at the step 531, the mobile station judges whether it has movesinto a cell of the neighboring base station or not by comparing thehighest neighboring base station receiving level and the currentreceiving level. When the highest neighboring base station receivinglevel is higher than the current receiving level, it is judged that themobile station has moved to a cell of the selected neighboring basestation at the step 531, and next at the step 532 the mobile stationswitches to the control channel of the selected neighboring base stationduring the idle period in the traffic channel and transmits thedestination base station notification signal at the located cellinformation notification time slot, whereas otherwise the step 530 isrepeated.

When the destination base station receives this destination base stationnotification signal in the control channel at the step 533, next at thestep 534, the destination base station outputs the control channelsignal of FIG. 22 to the loop transmission line, with itself as thegenerating station, the original base station as the addressed basestation, and the control signal indicating the occurrence of thehandover and commanding the release of the currently used trafficchannel.

When the original base station receives this control channel signalthrough the loop transmission line at the step 535, the original basestation releases the currently used traffic channel at the step 536 soas to stop relaying the communication of the mobile station.

Then, at the step 537, the destination base station selects an idletraffic channel available, and transmits the handover command signalindicating the idle traffic channel for the handover to the mobilestation by using the currently used traffic channel.

When the mobile station receives this handover command signal from thedestination base station at the step 538, the mobile station switchesits traffic channel to the idle traffic channel of the destinationstation at the step 539, while the destination base station startsrelaying the communication of the mobile station by assigning the idletraffic channel at the step 540.

The third procedure shown in FIG. 25 is for a case in which a commoncontrol channel is shared by all the base stations.

In this third procedure, first at the step 550, the mobile stationmeasures the current receiving level of the currently used base stationthrough which it is communicating at the mobile station reception timeslot, while measuring the receiving levels of the control channels ofthe neighboring base stations by switching to the control channel duringthe idle periods in the traffic channel and determining the selectedneighboring base station which has the highest neighboring base stationreceiving level.

Then, at the step 551, the mobile station judges whether it has movesinto a cell of the neighboring base station or not by comparing thehighest neighboring base station receiving level and the currentreceiving level. When the highest neighboring base station receivinglevel is higher than the current receiving level, it is judged that themobile station has moved to a cell of the selected neighboring basestation at the step 551, and next at the step 552 the mobile stationswitches to the common control channel of the base stations during theidle period in the traffic channel and transmits the destination basestation notification signal at the located cell information notificationtime slot, whereas otherwise the step 550 is repeated.

When the original base station receives this destination base stationnotification signal in the control channel at the step 553, the originalbase station releases the currently used traffic channel at the step 554so as to stop relaying the communication of the mobile station.

Meanwhile, when the destination base station receives this destinationbase station notification signal in the control channel at the step 555,next at the step 556, the destination base station selects an idletraffic channel available, and transmits the handover command signalindicating the idle traffic channel for the handover to the mobilestation by using the currently used traffic channel.

When the mobile station receives this handover command signal from thedestination base station at the step 557, the mobile station switchesits traffic channel to the idle traffic channel of the destinationstation at the step 558, while the destination base station startsrelaying the communication of the mobile station by assigning the idletraffic channel at the step 559.

The fourth procedure shown in FIG. 26 is carried out as follows.

First at the step 560, the mobile station measures the current receivinglevel of the currently used base station through which it iscommunicating at the mobile station reception time slot, while measuringthe receiving levels of the control channels of the neighboring basestations by switching to the control channel during the idle periods inthe traffic channel and determining the selected neighboring basestation which has the highest neighboring base station receiving level.

Then, at the step 561, the mobile station judges whether it has movesinto a cell of the neighboring base station or not by comparing thehighest neighboring base station receiving level and the currentreceiving level. When the highest neighboring base station receivinglevel is higher than the current receiving level, it is judged that themobile station has moved to a cell of the selected neighboring basestation at the step 561, and next at the step 562 the mobile stationtransmits the destination base station notification signal by usingeither the control channel or the currently used traffic channel of theoriginal base station, whereas otherwise the step 560 is repeated.

When the original base station receives this destination base stationnotification signal in the control channel or the currently used trafficchannel at the step 563, the original base station outputs the mobilestation signal of FIG. 21 to the loop transmission line, with itself asthe relay base station, and the control signal indicating the handovercommand at the step 564, and then releases the currently used trafficchannel at the step 565 so as to stop relaying the communication of themobile station.

When the destination base station receives this mobile station signalthrough the loop transmission line at the step 566, next at the step567, the destination base station selects an idle traffic channelavailable, and transmits the handover command signal indicating the idletraffic channel for the handover to the mobile station by using thecurrently used traffic channel.

When the mobile station receives this handover command signal from thedestination base station at the step 568, the mobile station switchesits traffic channel to the idle traffic channel of the destinationstation at the step 569, while the destination base station startsrelaying the communication of the mobile station by assigning the idletraffic channel at the step 570.

The fifth procedure shown in FIG. 27 is carried out as follows.

First at the step 580, the mobile station measures the current receivinglevel of the currently used base station through which it iscommunicating at the mobile station reception time slot, while measuringthe receiving levels of the control channels of the neighboring basestations by switching to the control channel during the idle periods inthe traffic channel and determining the selected neighboring basestation which has the highest neighboring base station receiving level.

Then, at the step 581, the mobile station judges whether it has movesinto a cell of the neighboring base station or not by comparing thehighest neighboring base station receiving level and the currentreceiving level. When the highest neighboring base station receivinglevel is higher than the current receiving level, it is judged that themobile station has moved to a cell of the selected neighboring basestation at the step 581, and next at the step 582 the mobile stationtransmits the destination base station notification signal by usingeither the control channel or the currently used traffic channel of theoriginal base station, whereas otherwise the step 580 is repeated.

When the original base station receives this destination base stationnotification signal in the control channel or the currently used trafficchannel at the step 583, the original base station outputs the idletraffic channel report command signal to the loop transmission line atthe step 584.

When the destination base station receives this idle traffic channelreport command signal through the loop transmission line at the step585, next at the step 586, the destination base station selects an idletraffic channel available, and transmits the idle traffic channel reportsignal indicating the idle traffic channel for the handover to the looptransmission line.

When the original base station receives this idle traffic channel reportsignal through the loop transmission line at the step 587, the originalbase station transmits the handover command signal indicating thereported idle traffic channel of the destination base station to themobile station at the step 588 by using the currently used trafficchannel at the step 588, and then releases the currently used trafficchannel at the step 589 so as to stop relaying the communication of themobile station.

When the mobile station receives this handover command signal from theoriginal base station at the step 590, the mobile station switches itstraffic channel to the idle traffic channel of the destination stationat the step 591, while the destination base station starts relaying thecommunication of the mobile station by assigning the idle trafficchannel at the step 592.

It is to be noted that this fifth embodiment is equally applicable to acase using a method other than TDMA, such as FDMA (frequency divisionmultiple access) and CDMA (code division multiple access). In usingFDMA, the monitoring of the receiving levels of the neighboring basestations can be achieved by using the method disclosed in JapanesePatent Application No. 61-192284. In using CDMA, the channels in theabove description for the case of using TDMA should be replaced bycodes.

Thus, according to the fifth embodiment, it becomes possible toestablish a so called decentralized autonomous control in the cellularsystem in which a mobile station monitors the receiving levels of thebase stations, judges whether it is moving into a cell of theneighboring base station, and transmits the located cell information tothe base stations In order to initiate the handover operation, withoutany command from the central control station, such that the handoveroperation can be carried out less time consumingly compared with aconventional cellular system without increasing the capacities and sizesof each base station and the central control station and complicatingthe configurations of the base stations and the central control station.As a consequence, it becomes possible to use the smaller cell radius soas to improve the frequency spectrum utilization efficiency, and tosecure the high quality of service regardless of the moving speed of themobile station.

Referring now to FIG. 28, a sixth embodiment of a mobile radiocommunication system using a method of route diversity according to thepresent invention will be described in detail.

In this sixth embodiment, the system comprises a central control station621 and a plurality (seven in FIG. 28) of base stations 622 to 628,which are connected through a inter-station transmission line formed bya transmission line element 629 connecting between the base stations 622and 623, a transmission line element 630 connecting between the basestations 623 and 624, a transmission line element 631 connecting betweenthe base stations 624 and 625, a transmission line element 632connecting between the base stations 625 and 626, a transmission lineelement 633 connecting between the base stations 626 and 627, atransmission line element 634 connecting between the base stations 627and 628, and a transmission line element 635 connecting between the basestation 628 and the central control station 621.

Each of the base stations 622 to 628 has a schematic configuration shownin FIG. 29, where the base station comprises an antenna 636 forreceiving the radio waves from a mobile station, a receiver 637 forreceiving the transmitted signal from the antenna 626, a control circuit640 connected with the receiver 637 through a received signal line 638and a receiving level information line 639, a transmission line element641 for receiving signals from an upper station through theinter-station transmission line, and a transmission line element 642 fortransmitting signals to a lower station through the inter-stationtransmission line.

This system of the sixth embodiment operates according to a diagramshown in FIG. 30 as follows.

The base station 622 is located at a top position of the inter-stationtransmission line, so that there is no signals from the upper stationfor this base station 622. Therefore, when this base station 622receives a signal 643 from a mobile station as a received signal 644,the receiver 637 of the base station 622 measures the receiving level ofthis received signal 644 at a measurement timing 645, and the measuredreceiving level is transmitted to the control circuit 640 through thereceiving level information line 639 while the received signal 644 istransmitted to the control circuit 640 through the received signal line638. At the control circuit 640, a signal transmission timing 650 fortransmitting the signal to the lower station is set to be equal to orsomewhat longer than the measurement timing 645, and the control circuit640 outputs an encoded receiving level information 648 indicating thereceiving level measured by the receiver 637 and a transmission signal649 identical to the received signal 644 together to the inter-stationtransmission line through the transmission line element 642.

At each of the remaining base stations 623 to 628, when the signal 643from a mobile station is received as the received signal 644, thereceiver 637 of the base station measures the receiving level of thereceived signal 644 at the measurement timing 645, and the measuredreceiving level is transmitted to the control circuit 640 through thereceiving level information line 639 while the received signal 644 istransmitted to the control circuit 640 through the received signal line638. Meanwhile, at the control circuit 640, the signal including thereceiving level information 646 and the transmission signal 647 isreceived from the upper station through the inter-station transmissionline and the transmission line element 641. The control circuit 640 thencompares the receiving level measured by the receiver 637 and thereceiving level indicated by the receiving level information 646.

When the receiving level measured by the receiver 637 is higher than thereceiving level indicated by the receiving level information 646, thecontrol circuit 640 outputs the encoded receiving level information 648indicating the receiving level measured by the receiver 637 and thetransmission signal 649 identical to the received signal 644 received bythe receiver 637 together to the inter-station transmission line throughthe transmission line element 642 at the signal transmission timing 650.

On the other hand, when the receiving level indicated by the receivinglevel information 646 is higher than the receiving level measured by thereceiver 637, the control circuit 640 outputs the encoded receivinglevel information 648 identical to the receiving level information 646and the transmission signal 649 identical to the transmission signal 647together to the inter-station transmission line through the transmissionline element 642 at the signal transmission timing 650.

Thus, the receiving level information 648 and the transmission signal649 to be transmitted to the lower station are sequentially updated atthe base stations 623 to 628, such that when the central control station621 receives the receiving level information 646 and the transmissionsignal 647 from the last base station 628 through the inter-stationtransmission line, the receiving level indicated by the receiving levelinformation 646 is the highest receiving level obtained among the basestations 622 to 628, and the transmission signal 647 represents thesignal received from the mobile station by one of the base stations 622to 628 at this highest receiving level, so that the route diversityreception is achieved by using the transmission signal 647 received bythe central control station 621 as the signal received from the mobilestation in the system.

Now, the signal transmission timing 650 will be described in furtherdetail.

Namely, when the base stations 622 to 628 are sequentially numbered byorders of positions on the interstation transmission line in a directionof the signal transmission, the signal transmission timing T₅,i for thei-th base station is given by:

    T.sub.5,i ≧T.sub.5,i-1 +T.sub.m +ΔT.sub.r

where T_(m) is a time required for receiving the receiving levelinformation 646 transmitted from the upper station, and ΔT_(r) is a sumof a radio propagation delay difference due to the difference of thedistances between the mobile station and the base stations 622 to 628and a signal transmission delay between the neighboring base stations onthe inter-station transmission line.

The radio propagation delay difference compensates the variation of thereceiving time of the radio waves from the mobile station at the basestations due to the difference of the distances between the mobilestation and the base stations 622 to 628, and can be considered as apropagation time for a distance between the neighboring base stations onthe inter-station transmission line, so that when the distance betweenthe neighboring base stations is 10 km for example, the radiopropagation delay difference is equal to 10×10³ /(3×10⁸)=3×10⁻⁵ sec=0.03ms.

As for the signal transmission delay between the neighboring basestations on the Inter-station transmission line, considering the factthat the transmission line is not straight, it is sufficient for thissignal transmission delay to take a value equal to several times theradio propagation delay difference, so that ΔTr=0.2 ms to 0.3 ms issufficient for the above described exemplary case.

Now, the operation of the system of the sixth embodiment described sofar assumes that all the base stations 622 to 628 can receive the signalfrom the mobile station. However, in reality, only a part of the basestations 622 to 628 will be able to receive the signal from the mobilestation in general.

In a case only a part of the base stations 622 to 628 can receive thesignal from the mobile station, the system of the sixth embodimentoperates as follows.

Let the J-th base station be the earliest base station in the order ofpositions along the inter-station transmission line which can receivethe signal from the mobile station. When the signal from the mobilestation is received at this J-th base station, the control circuit 640of this J-th base station will wait for the receiving level information646 and the transmission signal 647 from the upper station. However, inthis case, no upper station can transmit the receiving level information646 and the transmission signal 647 to the inter-station transmissionline because no upper station is capable of receiving the signal fromthe mobile station. Thus, after waiting for the signal transmissiontiming 650, the control circuit 640 of the J-th base station outputs thereceiving level information 648 indicating the receiving level measuredby the receiver 637 and the transmission signal 649 identical to thereceived signal 644 received by the receiver 637 to the inter-stationtransmission line through the transmission line element 642.

On the other hand, let the k-th base station be the base station whoseupper station is capable of receiving the signal from the mobile stationbut which itself is not capable of receiving the signal from the mobilestation. At this k-th base station, the receiving level information 646and the transmission signal 647 are received from the upper stationthrough the inter-station transmission line and the transmission lineelement 641 while not receiving the signal from the mobile station atthe receiver 637. Thus, after waiting for Tm+ΔTr since the reception ofthe receiving level information 646 and the transmission signal 647 fromthe upper station, the control circuit 640 of this k-th base stationoutputs the receiving level information 648 identical to the receivinglevel information 646 and transmission signal 649 identical to thetransmission signal 647 to the inter-station transmission line and thetransmission line element 642.

In this sixth embodiment, the level fluctuation can be averaged out byusing the longer receiving level measurement timing 645 such that theaccuracy of the receiving level measurement can be improved. However,the use of the longer receiving level measurement timing also makes thetransmission time on the transmission line longer so that thetransmission delay may present some problem in such a case.

Thus, the receiving level measurement timing 645 should be determined bytaking the maximum allowable transmission delay of the system intoaccount. In a case the maximum allowable transmission delay issufficiently large, the receiving level measurement timing 645 can beset equal to the entire signal duration, whereas in a case the maximumallowable transmission delay is not so large, the receiving levelmeasurement timing 645 should be set equal to only a part of a topportion of the entire signal duration.

Also, in this sixth embodiment, the receiving level information 648 andthe transmission signal 649 may be transmitted through the inter-stationtransmission line separately. In such a case, the receiving levelinformations from a plurality of the base stations may be multiplexedand transmitted by using a channel different from a channel used fortransmitting the transmission signals.

Thus, according to the sixth embodiment, it is possible to provide amethod of route diversity in a mobile radio communication which can berealized less expensively, without requiring additional control functionwhich complicates the central control station.

It is to be noted that in the above embodiments, the central controlstation and the base stations are provided separately, but the presentinvention is equally applicable to a system in which one of the basestations plays the role of the central control station as well.

Moreover, in the above embodiments, the receiving level is utilized asthe key for determining the destination base station, but the othersignal reception characteristics such as a number of detected errors oran eye opening in a case of digital signal with error correctionencoding, an S/N ratio in a case of analog signal, and theircombinations may also be utilized as the key for determining thedestination base station.

Besides these, many modifications and variations of the aboveembodiments may be made without departing from the novel andadvantageous features of the present invention. Accordingly, all suchmodifications and variations are intended to be included within thescope of the appended claims.

What is claimed is:
 1. A method of handover in a mobile radiocommunication using a cellular system formed by a plurality of basestations, in ring topology the method comprising the steps of:providinga single directional loop transmission line by which each base stationis connected with neighboring base stations; transmitting a handoverinformation through the loop transmission line, where the handoverinformation is relayed by each base station from one of the neighboringbase stations to another one of the neighboring base station such thatthe handover information is circulated through the loop transmissionline in a predetermined direction; and carrying out the handover of acommunication of a mobile station from one traffic channel of one basestation to an idle traffic channel of another base station by using thehandover information circulated through the loop transmission line. 2.The method of claim 1, wherein the handover information includes a basestation identification of a currently used base station, a currentlyused traffic channel at the currently used base station, and a receivingquality for radio waves from the mobile station at the currently usedbase station.
 3. The method of claim 2, wherein the receiving qualityincludes at least one of a receiving level, a number of detected errors,an eye pattern opening, and an S/N ratio.
 4. The method of claim 2,wherein each of the neighboring base stations of the currently used basestation recognizes the currently used base station according to the basestation identification of the currently used base station specified inthe handover information, and measures a receiving quality for the radiowaves from the mobile station in the currently used traffic channel atsaid each of the neighboring base stations according to the currentlyused traffic channel specified in the handover information.
 5. Themethod of claim 4, wherein each of the neighboring base stations of thecurrently used base station detects a motion of the mobile station froma cell of the currently used base station to a cell of said each of theneighboring base stations when the receiving quality measured by saideach of the neighboring base stations is greater than the receivingquality at the currently used base station specified in the handoverinformation.
 6. The method of claim 5, wherein the handover informationfurther includes a base station identification of one of the neighboringbase stations of the currently used base station which detected themotion of the mobile station from the cell of the currently used basestation to the cell of said one of the neighboring base station, and anidle traffic channel available for the handover at said one of theneighboring base stations.
 7. The method of claim 6, wherein thecurrently used base station commands the mobile station to carry out thehandover from the currently used traffic channel to the idle trafficchannel of said one of the neighboring base station of the currentlyused base station which detected the motion of the mobile station to thecell of the currently used base station to the cell of said one of theneighboring base station, according to the idle traffic channelspecified in the handover information.
 8. The method of claim 2, whereineach base station measures a receiving quality for the radio waves fromthe mobile station in the currently used traffic channel at said eachbase station according to the current used traffic channel specified nthe handover information.
 9. The method of claim 8, wherein the handoverinformation further includes a measuring base station identification ofa base station which measured the receiving quality specified in thehandover information, and when the receiving quality measured by eachbase station is greater than the receiving quality specified in thehandover information, said each base station replaces the measuring basestation identification in the handover information by a base stationidentification of said each base station and replaces the receivingquality in the handover information by the receiving quality measured bysaid each base station while relaying the handover information to saidanother one of the neighboring base stations through the looptransmission line, whereas otherwise said each base station relays thehandover information to said another one of the neighboring basestations through the loop transmission line without changing thehandover information transmitted to said each base station from said oneof the neighboring base stations.
 10. The method of claim 9, wherein thehandover information further includes an idle traffic channel availablefor the handover at a base station specified by the measuring basestation identification in the handover information.
 11. The method ofclaim 10, wherein the currently used base station commands the mobilestation to carry out the handover from the currently used trafficchannel to the idle traffic channel of the base station specified by themeasuring base station identification in the handover information,according to the idle traffic channel specified in the handoverinformation.
 12. The method of claim 1, wherein the cellular systemincludes a central control station for controlling the base stationswhich is connected to the loop transmission line, and a plurality ofland transmission lines connecting the central control station with eachof the base stations.
 13. The method of claim 12, wherein the handoverinformation includes a base station identification of a currently usedbase station, a currently used traffic channel at the currently usedbase station, base station identifications of the neighboring basestations of the currently used base station, and receiving qualities forradio waves from the mobile station measured at the neighboring basestations.
 14. The method of claim 13, wherein each of the neighboringbase stations of the currently used base station recognizes thecurrently used base station according to the base station identificationof the currently used base station specified in the handoverinformation, measures a receiving quality for the radio waves from themobile station in the currently used traffic channel according to thecurrently used traffic channel specified in the handover information,and outputs the base station identification of said each of theneighboring base stations and the receiving quality measured at saideach of the neighboring base stations as a part of the handoverinformation to the loop transmission line.
 15. The method of claim 14,wherein the central control station selects one of the neighboring basestations for which the measured receiving quality is the highest as adestination base station for the handover, and controls the basestations to carry out the handover through the land transmission lines.16. The method of claim 12, the handover information includes a basestation identification of each base station and an idle traffic channelavailable for the handover at said each base station.
 17. The method ofclaim 16, wherein the central control station selects one of the basestations as a destination base station for the handover by using theland transmission lines, and a currently used base station commands themobile station to carry out the handover from a currently used trafficchannel tot he idle traffic channel of the base station selected by thecentral control station as the destination base station according to theidle traffic channel of said destination base station specified in thehandover information.
 18. A method of handover in a mobile radiocommunication using a cellular system formed by a plurality of controlbase stations each of which is covering a cell within a service area ofthe system, a plurality of base stations each of which is covering amini-cell within a cell area of the cell covered by each of the controlbase stations, wherein the plurality of base stations within a cell areaof a control base station and the control base station form a ringtopology, method comprising the steps of:providing a single directionalloop transmission line by which a control base station and each basestation are connected with neighboring base stations; transmitting ahandover information through the loop transmission line, where thehandover information is relayed by each base station from one of theneighboring base stations to another one of the neighboring base stationsuch that the handover information is circulated through the looptransmission line in a predetermined direction; and carrying out thehandover of a communication of a mobile station from one traffic channelof one base station to an idle traffic channel of another base stationby using the handover information circulated through the looptransmission line when a mobile station moves from one mini-cell toanother mini-cell.
 19. The method of claim 18, wherein the handover fromone traffic channel of one of the base stations to an idle trafficchannel of the control base station is carried out when the mobilestation moves out of cell areas of the mini-cells within the cellcovered by the control base station, and when a receiving quality forradio waves from the mobile station at said one of the base stations islower than a functional capacity of said one of the base stations. 20.The method of claim 18, wherein the handover from one traffic channel ofone control base station to an idle traffic channel of another controlbase station is carried out when the mobile station moves out of thecell covered by said one control base station, under a control by asystem control station connected with each of the control base stationsthrough a land transmission line.
 21. A method of handover in a mobileradio communication using a cellular system formed by a plurality ofbase stations, in ring topology the method comprising the stepsof:providing a loop transmission line by which each base station isconnected with neighboring base stations; monitoring at a mobile stationa receiving quality of each of the base stations via a control channelduring idle periods of a currently used traffic channel and determininga destination base station for the handover; transmitting a handoverinformation through the loop transmission line, where the handoverinformation is relayed by each base station from one of the neighboringbase stations to another one of the neighboring base station such thatthe handover information is circulated through the loop transmissionline in a predetermined direction; and carrying out the handover of acommunication of the mobile station from the currently used trafficchannel of a currently used base station to an idle traffic channel ofthe destination base station determined by the mobile station by usingthe handover information circulated through the loop transmission line.22. The method of claim 21, wherein the handover information includesthe idle traffic channel available for the handover at the destinationbase station, and the currently used base station commands the mobilestation to carry out the handover from the currently used trafficchannel of the currently used base station to the idle traffic channelof the destination base station according to the idle traffic channelspecified in the handover information, while releasing the currentlyused traffic channel.
 23. The method of claim 21, wherein the handoverinformation includes a command for releasing the currently used trafficchannel such that the currently used base station releases the currentlyused traffic channel according to the command in the handoverinformation, while the destination base station commands the mobilestation to carry out the handover from the currently used trafficchannel of the currently used base station to the idle traffic channelof the destination base station.
 24. The method of claim 21, wherein thehandover information includes a notification of the destination basestation for the handover, and the destination base station commands themobile station to carry out the handover from the currently used trafficchannel of the currently used base station to the idle traffic channelof the destination base station in response to the notification in thehandover information, while the currently used base station releases thecurrently used traffic channel.
 25. The method of claim 21, wherein thehandover information includes a notification of the destination basestation for the handover and the idle traffic channel available for thehandover at the destination base station, and the currently used basestation commands the mobile station to carry out the handover from thecurrently used traffic channel of the currently used base station to theidle traffic channel of the destination base station according to theidle traffic channel specified in the handover information, whilereleasing the currently used traffic channel according to thenotification in the handover information.